Unfilled Rubber Research Articles

Исследование механического поведения эластомерных нанокомпозитов при больших циклических сдвиговых деформациях

The paper presents the results of experimental studies of viscoelastic properties of elastomeric nanocomposites based on styrene-butadiene rubber and various nanofillers: carbon black, detonation nanodiamonds, graphene nanoplates and single-walled graphene nanotubes. Corresponding tests were also carried out for pure unfilled rubber (as a basis for comparison and analysis of results). Non-standard samples and equipment specially designed for testing at finite shear strains (deformation by hundreds of percent) were used. In order to reveal the specific features of the viscoelastic behavior of the materials under consideration, test programs with complex loading trajectories were carried out — symmetric cyclic shear deformation with increasing amplitude and stops for relaxation when the direction of the tensile machine grips movement is changed. In addition, to evaluate the strength properties of these nanocomposites, they were all subjected to standard uniaxial tensile to rupture tests at a constant strain rate. The analysis of the obtained experimental data is carried out. It was found that the greatest enhancement effect is provided by the input of carbon nanotubes — several times in comparison with other samples. In this case, the largest increase in the hysteresis loop (i.e. dissipative loss) is also observed. This circumstance is very important when using such materials as damping elements. According to the authors, the effect of mechanical enhancement at large shear strains depends on the shape of the nanofiller particles. Nanotubes, due to their huge (compared to the transverse dimensions) length and flexibility, have a much stronger effect on the rearrangement of the composite microstructure compared to other types of fillers, thereby increasing dissipative losses

Characterization and modeling of temperature effect on the shear mode properties of magnetorheological elastomers

The temperature dependency of the viscoelastic properties of magnetorheological elastomers (MREs) operating in the shear mode was experimentally investigated under broad ranges of strain amplitude, excitation frequency, and applied magnetic field. Experiments were performed with isotropic MRE samples with volume fraction of carbonyl iron particles dispersed in the silicone rubber matrix under controlled temperature, ranging from to together with different levels of magnetic field density ( to ). The results revealed significant effect of temperature on the linear and nonlinear viscoelastic properties of MREs, in addition to the effects of strain amplitude, rate, and applied magnetic flux density. The temperature dependency of mechanical properties of an unfilled rubber matrix was also measured, which served as a reference. The shear properties of the MRE revealed reductions in both the storage and loss moduli with increasing temperature. The temperature dependency of the MRE, however, diminished under shear strain in excess of the critical strain. The results also revealed more pronounced temperature effect at higher temperatures and excitation frequencies, while it was relatively lesser with increasing magnetic field. Increase in temperature resulted in enhanced magnetorheological effect, while a clear trend with regard to the excitation frequency could not be established. Finally, a phenomenological model has been developed to predict the storage and loss moduli of the MRE as a function of excitation frequency, applied magnetic flux density, and temperature. The superb performance of the proposed model in predicting viscoelastic moduli under various operating and environmental conditions has been demonstrated through comparison of the experimental and simulation results.

ENTANGLED NETWORK INFLUENCED BY CARBON BLACK IN SOLUTION SBR VULCANIZATES REVEALED BY THEORY AND EXPERIMENT

ABSTRACTThe contribution of carbon black (CB) on changes in cross-link density (n) and physical entanglement has been quantified by swelling and uniaxial stress–strain measurements considering Mooney–Rivlin parameters. Solution SBR (SSBR) vulcanizates with varying content of high abrasion furnace (HAF) CB were studied. Rubber–filler networks increase the n values, which were determined by using Flory–Rehner and modified Guth–Gold equations by equilibrium swelling study. The Mooney–Rivlin parameter C1 was quantified using n, whereas parameter C2, representing physical entanglement, was determined for filled rubber by correlating with tensile results. The parameter C2 is monitored to be decreased with increasing CB (HAF) loading. A simple parabolic trend for physical entanglement parameter with increasing CB loading is proposed, and it shows a correlation coefficient (R2) of 0.99595. Atomic force microscopy study confirms the generation of filler networking in the rubber matrix. The current findings elucidate a way for quantifying physical network changes due to fillers in an unfilled rubber system.

The properties of unfilled natural rubber in the existence of alkanolamide: swelling, mechanical and morphological properties

A study was carried out on the utilization of alkanolamide as a new curative additive in natural rubber (NR) compounds by using a semi-efficient vulcanization system. The alkanolamide was derived from the waste of cooking oil and incorporated into the unfilled-NR compound at 0.2, 0.4, 0.6, 0.8, and 1.0 phr. The effect of alkanolamide on swelling percentage, mechanical and morphological properties of the NR compounds were observed. It was found that the addition of the alkanolamide increased elongation at break of the NR compounds. The alkanolamide also increased tensile modulus, tensile strength, hardness and crosslink density of up to 0.6 phr of alkanolamide loading and then decreased with further increases in the alkanolamide loading. Scanning electron microscopy (SEM) micrographs proved that 0.6 phr loading of alkanolamide in the NR compounds exhibited the greatest matrix tearing line and surface roughness which correlated to a higher tensile strength of the NR compounds.

The properties of unfilled natural rubber in the existence of alkanolamide: rheological and crosslink density properties

Using a semi-efficient vulcanization system; a study was carried out on the utilization of alkanolamide as a curative additive in natural rubber (NR) compounds. The alkanolamide was incorporated into the unfilled-NR compound at 0.2, 0.4, 0.6, 0.8, and 1.0 phr. The effect of alkanolamide on rheological and crosslink density properties of the NR compounds were observed. It was found that the addition of the alkanolamide exhibited shorter scorch and cure times and higher elongation at break of the NR compounds. The alkanolamide also exhibited higher torque differences and crosslink density of up to 0.6 phr of alkanolamide loading and then decreased with further increases in the alkanolamide loading.

The Potential of Oil Palm Ash and Eggshell Powder as Hybrid Biofillers in Natural Rubber Biocomposites

Astronomical amounts of solid biomass wastes are generated globally each day. It is imperative that the potential of conversion and utilisation of these wastes be identified and implemented, thus reducing landfill load and recovering value from this immense source of material. Natural rubber biocomposites were produced by compounding natural rubber matrix with oil palm ash and eggshell powder as hybrid fillers. The mechanical properties of the biocomposites were determined. It is observed that the biocomposites exhibit higher stiffness than the unfilled natural rubber compound. Acid treatment is found to further increase the stiffness of the biocomposites.

Comparison between x-ray diffraction and quantitative surface calorimetry based on infrared thermography to evaluate strain-induced crystallinity in natural rubber.

The crystallinity of stretched crystallizable rubbers is classically evaluated using x-ray diffraction (XRD). As crystallization is a strongly exothermal phenomenon, quantitative surface calorimetry from infrared thermography offers an interesting alternative to XRD for determining the crystallinity. In this paper, the two measurement techniques have been used for evaluating the strain-induced crystallinity of the same unfilled natural rubber. This study provides the first comparison between the two techniques. The results obtained highlight the very satisfactory agreement between the two measurements, which opens a simple way for evaluating the strain-induced crystallinity from temperature measurements.

Natural rubber composites with Grits waste from cellulose industry

Tons of industrial residues are daily generated and require management strategies to be correctly disposed or recycled. Therefore, the reuse of organic and industrial wastes as fillers in polymeric composites has gained attention as a new research field. Here, it is demonstrated for the first time the use of Grits waste as filler in natural rubber composites to reuse the eucalyptus kraft pulp residue as well as to decrease the final cost of rubber products by totally or partially replacing the commercial fillers. Untreated waste was employed without pH correction and any coupling agent, while the treated waste had a pH correction and treatment with Dodigen 1611 which is a coupling agent formed by quaternary ammonium salt and increase the residue dispersion or linked it into the polymeric matrix. The pH correction was applied to avoid interference in the vulcanization reaction and it was performed with acetic acid 50% v/v until waste neutralization. Composites with 20 phr of untreated Grits have shown an increase, around 7%, in the stress values when compared to the unfilled natural rubber. However, composites with 20 phr of treated Grits had an increase of 20%. Abrasion loss results show that abrasion resistance tended to decrease when untreated and treated Grits contents are higher than10 phr. An alternative application for the Grits waste composites was in the production of slippers. The produced composites were able to be processed, cut, and molded in an industry of shoes and slippers. The slippers could be well implemented as the composite flexibility was preserved.

Modeling the non-dissipative response of vulcanized unfilled rubber

Here the possibility of developing a better three dimensional model for rubber when Hencky strain is written as an explicit function of the Cauchy stress is examined. Based on the experimental evidence, vulcanized unfilled rubber is assumed to be isotropic, incompressible, and deforming from a stress-free reference configuration. Vulcanized unfilled rubber undergoes strain-induced crystallization when uniaxial stretch exceeds 2. Therefore, in this study, a non-dissipative model is sought for rubber only till the von Mises stress is below a critical value. Using the biaxial response reported in Kawabata et al. (1981), the material functions are arrived adopting Rivlin and Saunders methodology. The material constants in the determined material functions are found using biaxial tests for four other experimental studies on rubber reported in the literature. The ability of the arrived constitutive model to describe the uniaxial and equal biaxial response of the same rubber is evaluated. The proposed model is benchmarked with established models in the classical framework. Among the models studied the proposed model performed as well as the other models for Kawabata et al. data set and marginally better than the other models for the remaining four experimental data sets. However, while the proposed model performance does not critically depend on the experiments used to deduce the material parameters, for the other models the material parameters need to be found from fixed biaxial experiments for better performance.

New observations regarding fatigue crack paths and their fracture surfaces in natural rubber: Influences of R-ratio and pre-load

Most previous works focusing on the influence of R-ratio on the service life of natural rubber didn’t consider some interesting phenomena. Cracks in both filled and unfilled natural rubber have the tendency to split and propagate into the stress relaxation zone (“backwards” at R = εmin/εmax = 1/3) with increasing R-ratio. When a “backwards” crack arose, regular ordered striations were observed on the fracture surface. Additionally, the crack growth behaviour of filled natural rubber can differ greatly from catastrophic crack growth to slow “backwards” fatigue growth, even when all loading conditions such as frequency, amplitude and R-ratio were kept constant.

Strain-induced crystallisation in natural rubber: a thermodynamically consistent model of the material behaviour using a multiphase approach

Strain-induced crystallisation (SIC) is the phenomenon of elastomers to experience a pronounced nonlinearity under large deformations of several hundred percentages of strain, which is advantageous for industrial applications due to the resulting positive properties such as increasing crack growth resistance and fatigue behaviour. The overall objective of the current study is the constitutive modelling of the material behaviour of natural rubber when stretched uniaxially. Initially, unfilled natural rubber is considered. Cyclic traction experiments in which crystallinity, elongation and stresses are simultaneously measured are found in the literature. The same vulcanisate is used to conduct additional experiments in the present laboratory. The experimental investigations focus on uniaxial tension tests, conducted with a variety of parameters such as time and stretch rate. The current work presents an approach to model the SIC phenomenon. The model considers thermoelasticity and crystallisation. The approach of the hybrid free energy is used to derive constitutive equations and to meet thermomechanical consistency. However, the effect of temperature is not the focus of the current work. Furthermore, the contribution of a mixing energy is represented in the free energy. The derivation of this mixing entropy is explained in detail by making use of different assumptions and approaches, e.g. the consideration of ideal mixtures. Here, a one-dimensional constitutive model for strain-crystallising rubber is developed, which can be extended with a micro-sphere approach (concept of representative directions) to a full thermodynamically consistent anisotropic model.

In situ dilatometry and X-ray microtomography study on the formation and growth of cavities in unfilled styrene-butadiene-rubber vulcanizates subjected to constrained tensile deformation

It is known, that under constrained tensile deformation the mechanical response and the failure behavior of rubber vulcanizates reveal unlikely features. Due to multiaxial stress concentrations, the formation of internal cracks can be initiated.For the first time, fundamental investigations on unfilled styrene-butadiene-rubber pancake specimens were performed. Via in situ dilatometry and X-ray microtomography experiments, the damage process under constrained tensile deformation is studied and discussed. Although cavitation has been often discussed for filler reinforced rubber vulcanizates, the results of this study demonstrate how it occurs also in unfilled rubbers. In fact, the stronger the geometrical constraints are, the higher is the number of small cavities. However, the integral cavity volume is not affected. Moreover, micrographs of fracture surfaces indicate that cavitation is controlled by an omnidirectional growth of radial side-cracks. Finally, an energy-based approach to describe the cavitation onset criterion initiating the damaging process is presented to revise Gent's theory.

Investigating nonlinear vibrations of higher-order hyper-elastic beams using the Hamiltonian method

This paper presents a higher-order shear deformation beam theory for modeling and nonlinear vibration analysis of hyper-elastic beams made of silicon rubber and unfilled natural rubber. Four models named neo-Hookean, Mooney–Rivlin, Ishihara, and Yeoh models are presented, and their efficacy in nonlinear dynamic modeling of hyper-elastic beams has been explored. Geometric nonlinearity of the hyper-elastic beam is considered based on von-Karman-type nonlinearity. The hyper-elastic beam is resting on a nonlinearly hardening elastic foundation. It is shown that the Ishihara model is a suitable model for nonlinear vibration analysis of hyper-elastic beams accounting for the shear deformation effect. The nonlinear governing equations based on the presented beam theory are analytically solved via the Hamiltonian method to find nonlinear vibration frequencies. It is shown that the nonlinear vibration behavior of hyper-elastic beams is influenced by rubber-material type and material parameters of the hyper-elastic model.

Effect of surface treatments and filler loading on the properties of hemp fiber/natural rubber composites

The aim of this present work was to study the effect of surface treatment methods, i.e., silane and permanganate on curing characteristics, dynamic mechanical, mechanical morphological and thermal properties of hemp fiber filled natural rubber composites. Hemp fiber surfaces were pretreated with an alkali solution and followed by a KMnO4 solution or (3-triethoxysilylpropyl) tetrasulfide (Si69). After that, the chemical, physical and morphological properties of untreated and treated hemp fiber samples were investigated by attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. It was found that the KMnO4 and silane (Si69) treatments affected the surface characteristics of hemp fibers. The curing characteristics of the rubber composites, i.e., scorch and curing times increased with increased fiber loading, except for 15 parts per hundred of rubber (phr) fiber level. The curing characteristics improved with the addition of KMnO4 or silane treated hemp fiber compared to unfilled natural rubber composites. At 5 phr fiber loading, it was found that silane treated hemp fiber filled natural rubber composite showed greater tensile strength and crosslink density in comparison with KMnO4 treated and untreated hemp fiber filled rubber composites. Good interface interaction between the silane treated hemp fiber and rubber matrix was observed in SEM images. Additionally, the thermal stability of rubber composites was unaltered with the incorporation of KMnO4 or silane treated hemp fibers.

Mechanical and Fire Properties of Multicomponent Flame Retardant EPDM Rubbers Using Aluminum Trihydroxide, Ammonium Polyphosphate, and Polyaniline.

In this study, multicomponent flame retardant systems, consisting of ammonium polyphosphate (APP), aluminum trihydroxide (ATH), and polyaniline (PANI), were used in ethylene propylene diene monomer (EPDM) rubber. The multicomponent system was designed to improve flame retardancy and the mechanical properties of the rubber compounds, while simultaneously reducing the amount of filler. PANI was applied at low loadings (7 phr) and combined with the phosphorous APP (21 phr) and the mineral flame retardant ATH (50 phr). A comprehensive study of six EPDM rubbers was carried out by systematically varying the fillers to explain the impact of multicomponent flame retardant systems on mechanical properties. The six EPDM materials were investigated via the UL 94, limiting oxygen index (LOI), FMVSS 302, glow wire tests, and the cone calorimeter, showing that multicomponent flame retardant systems led to improved fire performance. In cone calorimeter tests the EPDM/APP/ATH/PANI composite reduced the maximum average rate of heat emission (MARHE) to 142 kW·m−2, a value 50% lower than that for the unfilled EPDM rubber. Furthermore, the amount of phosphorus in the residues was quantified and the mode of action of the phosphorous flame retardant APP was explained. The data from the cone calorimeter were used to determine the protective layer effect of the multicomponent flame retardant systems in the EPDM compounds.

Studies on the Influence of Mixed Culture from Buried Soil Sample for Biodegradation of Sago Starch Filled Natural Rubber Latex Gloves

The addition of sago starch into natural rubber (NR) latex compounds was aimed to accelerate the degradation process of the end products including gloves as an alternative to reduce solid waste disposal problem. In this research work, the degradation of sago starch filled natural rubber (SS/NR) latex gloves by a mixed culture from buried soil sample was investigated. The Gram staining test of mixed culture showed the Gram-negative bacteria are the dominant species as an effective bacterium to degrade the SS/NR latex gloves. The starch hydrolysis test was performed to confirm the existence of starch-degrading bacteria in mixed culture that consume sago starch from the gloves. Based on the biodegradation rate results obtained, the existence of starch-degrading bacteria together with rubber-degrading bacteria were observed, which accelerate the biodegradation of SS/NR latex gloves by 53.68%. Meanwhile, the biodegradation rate for sago starch unfilled natural rubber (control) latex gloves were only 50.31%. Thus, the addition of sago starch has shown an increased in the biodegradation rate, which can be used as alternative method to overcome the solid waste disposal problem.

Effect of Different Types of Rubbers and Carbon Black Grades on Curing Characteristics and Dynamic Properties for Anti-vibration Application

Nowadays, the automotive industry is facing a major challenge to meet the changing demand on performance, material specifications and durability requirements. An anti-vibration part is a critical component in automotive, used to eliminate the vibration. In order to produce a good anti-vibration component, the rubber compound needs to be formulated with good damping properties. This research work focused on the effect of (unfilled) rubber types; natural rubber (SMR 60CV, SMR L, RSS) and synthetic rubber (SBR, NBR, CR, EPDM) and the effect of carbon black grades (N220, N330, N550, N774, N990). The curing characteristics and dynamic properties (dynamic stiffness, Kd) at 15, 45 and 100 Hz were analyzed. The curing characteristic results showed there is a relationship between both factors. The longer cure time for types of rubber indicates longer time is needed to break higher bond energy of the rubber chain. Meanwhile, for carbon black grades, the cure rate is gradually increased with lower carbon black surface area due to the diffusion limitation and lower chain mobility. The dynamic properties results were based on the Payne effect theory which influenced by the rubber bond structure that gave higher value of dynamic stiffness due to rubber-rubber interaction. For carbon black grades, the higher surface area of carbon black gave higher dynamic stiffness due to increased rubber-filler and filler-filler interactions.

About the elongation at break of unfilled natural rubber elastomers

This paper presents the characterization of the mechanical properties, of the ability to crystallize thermally or under strain, and of the Elastically Active Chains (EAC) network of Natural Rubber elastomer, whose EAC average density and length distribution were modified by thermo-oxidative ageing. EAC length distribution is not a critical parameter for λbreak in elastomers which do not crystallize (as far as the quantity of free and dangling chains stays negligible, which is the case in this study). However, it becomes important in crystallizable materials, since it controls the chains quantity which can be stretched enough to crystallize. When this quantity is large enough, the material can be stretched up to λc, and the crystallization process will enable to increase both λbreakand σbreak. Conversely when it is too low, λbreakis seemingly mainly governed by the average EAC density and a power law then links both quantities.

INFLUENCE OF STRAIN-INDUCED CRYSTALLIZATION ON STRESS SOFTENING OF SULFUR CROSS-LINKED UNFILLED GUAYULE AND DANDELION NATURAL RUBBERS

ABSTRACT The stress softening behaviors of sulfur cross-linked unfilled guayule natural rubber (S-GR) and sulfur cross-linked unfilled dandelion natural rubber (S-DR) under cyclic deformation were characterized by simultaneous wide angle X-ray diffraction and tensile measurements under cyclic deformation. The behaviors were found to be affected by their strain-induced crystallization (SIC) phenomena and aggregated nonrubber components. The stress softening degree at stretching ratio = 8.0 of S-DR was almost two times larger than that of S-GR. Additionally, the hysteresis loss and residual strain of S-DR were much higher than those of S-GR in the same cycle. The key factors that caused the increase in their degree of stress softening were the increase in average volume and the decrease in average number of strain-induced crystallites upon cyclic deformation, not the insignificant decrease in crystallinity. The breakage of the aggregated nonrubber components is a main origin of stress softening behaviors for S-GR and S-DR because their SIC behaviors also were significantly influenced by the aggregates of nonrubber components in the rubber matrixes. Both the effects of nonrubber components and SIC on stress softening were more dominant in S-DR than in S-GR, probably owing to the larger amount of aggregated nonrubber components in the former than in the latter. The results will be useful in effectively using guayule and dandelion natural rubbers as alternatives to Hevea natural rubber in the rubber industry.

Influence of processing parameters on barrier properties of nitrile rubber/nanoclay nanocomposite membrane against organic solvent

The present study focuses on the influence of layered silicate nanoparticles on barrier properties of a rubber/nanoclay nanocomposite (RCN). Most of the previous works deal with the importance of the surface chemistry of the nanoclay on the performance of the material. In this paper were rather investigated the effect of the processing conditions. Elastomer nanocomposites based on nitrile butadiene rubber and layered silicate nanoparticles were prepared using three different techniques: chemical dissolution, mixing in an internal mixer and melt-mixing with a twin-screw extruder. Chemical resistance to methanol was assessed using gravimetric method and mass transfer kinetic parameters were then deduced. Mass uptake results clearly showed that extruded RCN exhibited the highest chemical resistance. Barrier properties were subsequently correlated to the morphological structure of the rubber/nanoclay system. Small-angle X-ray diffraction patterns indicated that this improvement was due to a preferred alignment of the nanoparticle in the structure. Further, inspection revealed an anisotropic orientation profile of the nanoclay layers over the thickness of a sheet-like extruded material. In addition, a neat decrease of chemical sorption was spotted with mixing and chemically processed RCN materials over the unfilled rubber. The state of dispersion of clay nanoparticles was also probed in these nanocomposites using X-ray diffraction. The findings suggested an intercalation during processing. Further, transmission electron microscopy observations were suggesting a bimodal state of dispersion over spotting individual nClay platelets along with the layered stacks. Improvement of mass transport kinetic parameters was recorded over increasing the shear stress as well as the residence time when RCN were prepared in the internal mixer.